Within the printing industry there is a variety of techniques known for transferring designs to paper, for example, by means of print originals. One possibility is that known as flexographic printing.
In the flexographic printing process, flexible printing plates are bonded to printing cylinders or printing sleeves. Such plates are composed, for example, of a polyethylene terephthalate film (PET film) to which is applied a layer of a photopolymer, into which the corresponding print relief can be introduced by exposure to light. In this case the bonding of the plate to the printing cylinder or printing sleeve is via the PET film.
For the bonding operation it is usual to use double-sided pressure-sensitive adhesive tapes, which are subject to very exacting requirements. For the printing operation, the pressure-sensitive adhesive tape is required to have a certain hardness, but also a certain elasticity. These properties must be set very precisely, so that the resulting printed image, in accordance with the requirements, delivers the desired result. Exacting requirements are likewise imposed on the pressure-sensitive adhesive, since the bond strength must likewise be sufficient for the printing plate not to detach from the double-sided pressure-sensitive adhesive tape, or the pressure-sensitive adhesive tape from the cylinder or sleeve. This applies even at increased temperatures of 40 to 60° C. and at relatively high printing speeds. Besides this characteristic, however, the pressure-sensitive adhesive is also required to posses reversible adhesion properties, in order to be able to detach again from the printing plates after the printing operations (for such redetachability, it must be possible to undo not only the adhesive bond of the pressure-sensitive adhesive tape to the printing cylinder or printing sleeve but also the adhesive bond to the plate, without residue, in order to ensure that both components can be used again). This detachability ought to exist even after bonding over a relatively long time period (up to 6 months). It is desirable, moreover, for the pressure-sensitive adhesive tape, and particularly the printing plate, to be removable without suffering destruction, in other words without great expenditure of force, since the printing plates are usually used more than once. Moreover, there should be no residues remaining on the printing plate or on the cylinder or sleeve. In summary, therefore, the requirements imposed on the double-sided pressure-sensitive adhesive tapes suitable for this utility are very exacting.
Pressure-sensitive adhesives (PSAs) contemplated include, for example, those based on natural rubber, as documented by EP 760 389 A, for instance. For the stated end utility, however, pressure-sensitive adhesive tapes are also used that comprise polyacrylate-based PSAs. For example, WO 03/057497 A describes a block copolymer-based acrylate PSA for the stated end application. WO 2004/067661 A discloses a pressure-sensitive adhesive tape featuring a PSA based on a soft acrylic monomer (Tg<−30° C.) to an extent of at least 49.5% by weight, at least one hard, cyclic or linear (meth)acrylic ester monomer (Tg≧30° C.) to an extent of at least 10% by weight, and also functionalized, hard (meth)acrylic acid/ester monomers (Tg≧30° C.) to an extent of at least 0.5% by weight, the PSA being prepared in a two-stage process. It is reported that the latter of the stated components, which includes acrylic acid, may be present in fractions of up to 10% by weight. There is no teaching, however, of how this component, if selected as acrylic acid, might be copolymerized in correspondingly high fractions with retention of the requisite pressure-sensitive adhesive properties, or of the extent to which high fractions of acrylic acid would affect the pressure-sensitive adhesive properties. Correspondingly, all of the examples given show an acrylic acid fraction of only 2% by weight in the PSA, corresponding to the expectations of the effect of hard monomers on the requisite application properties (on this point see later on below). The solvent resistance of the resulting materials is not addressed.
Associated with the PSAs known from the prior art, however, is a problem. Although they are indeed very suitable for an operationally reliable bond of the printing plates to printing cylinders and to printing sleeves, and have good redetachability thereafter, there is nevertheless failure of the adhesive bond in contact with solvents that may penetrate the bonded joint. This is the case particularly when the bonded printing plates are to be cleaned to remove the printers ink. The usual way of achieving this is to use the solvents, which also serve as solvents for the inks per se, in large quantities for the washing and detaching of the inks from the plates. Inevitably in such operations, however, there is an undermining of the edges of the bond of the plate on the pressure-sensitive adhesive tape. This causes detachment of the plates, since the adhesives of the pressure-sensitive adhesive tape lose the required adhesion to the plate. The flaps that this produces prevent further printing operations by causing the printing ink to smear, if not indeed leading to mechanical problems with the plate flaps in the printing apparatus and hence to system failures. In practice, therefore, printing plates that are mounted with prior-art adhesives have to be protected from the solvent by sealing of the plate edges with single-sided pressure-sensitive adhesive tapes or with liquid adhesives or hot melt adhesives.
This additional sealing operation entails a significant extra expense and effort, and harbours the risk of damaging the expensive printing plates on demounting, particularly when liquid or hot melt adhesives are used.
It therefore appears necessary to develop a pressure-sensitive adhesive which does not have the disadvantages described in the prior art and which accomplishes the described application without additional sealing of the plate edges.
It is an object of the invention to offer a pressure-sensitive adhesive which, even under the influence of solvents, ensures effective and secure bonding, particularly to PET (polyethylene terephthalate), and yet is redetachable even after a prolonged time. The pressure-sensitive adhesive is preferably to be suitable more particularly for the bonding of printing plates, with the printing plates having no tendency, or a significantly reduced tendency, towards edge lifting under the influence of solvents.